Contribution limits to retirement plans mean that for a billionaire, their 401k or IRA balance is very unlikely to be a significant portion of their portfolio. The typical case I can imagine for this would be putting their roughly $20,000 worth of 401k contributions into company stock every year and then seeing their company go 100x.
https://www.propublica.org/article/lord-of-the-roths-how-tec...
Peter Thiel managed to get a $5 billion IRA.
> [1]Prohibited transactions in a qualified plan
> 4. Any of the following acts between the plan and a disqualified person:
> [1] Disqualified person. You are a disqualified person if you are any of the following.
> 8. An officer, a director (or an individual having powers or responsibilities similar to those of officers or directors), a 10% or more shareholder, or a highly compensated employee (earning 10%-or-more of the yearly wages of an employer) of a person described in (3), (4), (5), or (7).
[1]: https://www.irs.gov/publications/p560#en_US_2022_publink1000...